This invention relates to magnetic resonance (MR) methods. More specifically, this invention relates to method and apparatus for reduction of acoustical noise generated when gradient coils used for generating magnetic field gradients in an MR scanner are energized in the presence of a polarizing magnetic field.
The magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositons. More recently, MR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in vivo, non-invasive spectroscopic analysis. As is now well known, the MR resonance phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or different RF coil is used to detect the NMR signals emanating from the patient volume lying within the field of the RF coil. In the course of a complete MR scan, a plurality of MR signals are typically observed. The signals are used to derive MR imaging or spectroscopic information about the object studied.
In typical studies, the MR signal is usually observed in the presence of pulsed linear magnetic field gradients used to encode spatial information into the signal. Pulsed magnetic field gradients are also employed with selective RF pulses to excite nuclear spins in predetermined regions of the object undergoing examination. In the course of an MR examination, it is frequently desirable to apply pulsed magnetic field gradients in each of the x, y, and z directions of a conventional Cartesian coordinate system. It will be recognized, however, that in practice the direction in which magnetic field gradient pulses may be applied is not limited in any manner, and any direction could be selected as required.
A typical gradient coil assembly utilized in an MR scanner comprises a fiber-glass coil form on which three gradient coils, utilized for generating the G.sub.x, G.sub.y and G.sub.z magnetic field gradients directed in the like-designated directions of the coordinate system, are wound. The fiber-glass coil form is selected to have a cylindrical configuration having a diameter sufficiently large to accommodate the torso of a patient in the case of a whole-body MR scanner. The complete gradient coil assembly is mounted in the bore of a magnet utilized for generating the homogeneous polarizing magnetic field. The magnet is frequently selected to be of superconductive design and is capable of generating magnetic fields in excess of 15,000 gauss. In operation, the gradient coils are energized with current pulses which may be of up to 70 amperes and having durations of approximately 3-4 milliseconds. Such current pulses create high forces in those portions of the gradient coil conductors that are perpendicular to the polarizing magnetic field. These forces create vibrations on the fiber-glass coil form that are in the audible range of approximately 63-10,000 Hz. Under such conditions, the vibrating walls of the coil form are capaable of generating high levels of acoustical noise reduction techniques, absorption, mass barriers and damping are effective, because of the noise levels involved, further reductions are desirable.
It is therefore a principal object of the invention to provide method and apparatus for achieving acoustical noise reductions beyond those achievable with conventional noise reduction techniques.